This invention relates to manufacture of semiconductor devices and more particularly to a target used in laser programming of such devices.
Redundant memory devices contain extra rows and columns of cells which are substituted when cells are faulty in the main array. The addresses of faulty rows and columns are programmed into a memory device, after testing, by a laser beam method. The laser beam is used to make or break certain connectors, depending upon the test results. In VLSI devices, the size of the lines or elements that the laser beam must focus upon are a few microns, or less. Thus, the beam must be accurately positioned as it scans a silicon slice. To this end, the beam is made to find some indexing structure or target as it enters each bar on the slice.
A major problem seen with laser scribing for redundant memory devices is the consistancy of response from these targets. The targets most widely used in production are made of a single strip of material. The width of the strip is generally larger than the spot size of the laser beam used to detect the target. The visibility of this target depends upon the difference in reflectivity between underlying layers and the target strip. The problem encountered with this type of target is that the contrast of the reflected image of the target may vary with slight differences in process. The changes in contrast cause the target image to appear to change position, disappear, or even change color (reflective becomes non-reflective and vice versa).
One target proposed by others to eliminate these problems was a single strip or valley of material much smaller than the spot size. For a 6-8 micron laser beam spot the target was 1-2 microns wide. For this type of target the problems described previously still remained, plus new problems were encountered; for example, a shift in the apparent position of the image due to false edge indications. Another problem introduced was a consequence of the small size of the target structure with respect to the laser spot size. The accuracy with which this prior target could be found diminished because of the uncertainties in locating the center of the small structure within the diameter of a large spot. Ultimately the smaller line target was found to be of no net advantage over a larger structure as the first target.
In particular, the larger target (currently most widely used) was made from a single strip of material that was the same as the link to be blown. The strip was typically about 80 microns in length and 12 microns wide. There was a 50 micron wide clear area around the strip. The target was uncovered during processing to allow the target material to be completely unobstructed by any overlaying level of material that could reduce the contrast of the tartet image. The design of the strip was such that the strip was larger in width than the spot size of the laser.
The smaller target was similar in construction to the first target with the exception that the line width was smaller. Typically, the line width for this target was 1-2 microns for a 6-8 micron size beam.
A third type of prior target consisted of a photo conductive target that used photon induced conductivity of a moat strip as the detection mechanism for the position of the laser beam. This third type of target was constructed by placing a material over the moat that would block the laser beam. As the laser beam was scanned over the structure the moat was conductive as long as the laser beam was over the moat area. As the laser beam moved on to the blocking material the conductivity of the moat was reduced. The change in conductivity of the moat with laser beam position defined an edge that the laser alignment system was to be referenced to.
All these targets described so far have been used for the 1.06 micron line of the Nd:YAG laser. The 0.53 micron of the Nd:YAG laser presents a special problem to the targets. Only the photo conductive target which uses photo induced currents seems to work at all for the 0.53 micron laser.
The purpose of the improved target of this invention is to resolve these problems and at the same time improve the laser accuracy. The new target is built in such a way as to be "process independent". The previous targets vary in quality due to changes in processing parameters from one side of the slice to the other. The process may also vary from one slice to another. In particular, the targets described previously were affected by the final metal etch and the various oxide thicknesses used to construct the device.
The target of the invention is suitable for any wave length of light in the laser beam as long as some simple design rules are followed. Furthermore, the target does not lose accuracy as the small sized target. In fact, the target has been found to be more accurate than the wide strip target mentioned above.
It is the principal object of this invention ot provide an improved method of laser beam programming of VLSI semiconductor devices, particularly an improved target for indexing a laser beam. Another object is to provide an indexing target which produces a constant and readily identifiable response independent of processing variables, materials, and the like.